U.S. Pat. No. 4,189,642 describes an insulated rotatable drum carrying a photoconductor supported on a conductive layer. The conductive support layer is connected via slip rings to ground, and when the circuit is opened, and each corona individually operated, current in the conductive support layer may be measured.
Having a drum of floating voltage, i.e., electrically insulated, would result in irregular charge patterns on the photoconductor except for a sheet of conductive backing for the photoconductor, which must be connected to ground. Further, to provide a measurement of each corona individually, when the conductive backing is adjacent all coronas, a control circuit must be provided to individually activate each corona. A control circuit must also open the ground connection for the conductive backing to allow measurement. The resultant arrangement is therefore very expensive and comprises a permanent part of every machine, whereas the measurement to be made is done only occasionally by or for a repair or maintenance technician.
Another arrangement often used in the assembly of copiers is to remove the normal photoconductor and substitute a special insulated drum with conductive areas for measurement purposes.
Special insulated drums or fixtures for testing are not permanent parts of each machine and are therefore less expensive as concerns total cost. However, they are extremely bulky and cannot be easily or conveniently transported by a repair or maintenance technician. Further, any slight difference in size between the test fixture and the photoconductor carrier, creating a difference between the position of measurement and the actual position of the photoconductor which is to receive a charge, would result in an error.